The light-transmission type smoke detector is well known. Detectors of this type require a rather long optical path to attain sufficient dimming of a light beam. In Japanese Laid-Open Utility Model Publication No. 61-10549, filed June 25, 1984 and published Jan. 22, 1986, is described a light-transmission type smoke detector having an optical path comprising a spiral tube formed in a disc-like block of a plastic material, the inside surface of which constitutes a mirror and which is provided with a plurality of small through holes, through which smoke may enter the tube. This detector has advantages and disadvantages. It is advantageous in that it has low susceptibility to electrical noise since its light-emitting and light-receiving elements are shielded by the metal film coating provided on the inside surface of the optical path to constitute a mirror. It is disadvantageous in that smoke does not easily enter the optical path through the small through holes.